Honeycomb monolith structures comprise a plurality of cell walls defining a plurality of channels, or elongated cells, separated from each other by said cell walls, wherein the plurality of cell walls and channels extend in parallel along a common direction from an entrance end to an outlet end of the structure in the fluid flow direction. The channels are open in both ends. Monolith structures are usually given a complementary shape and placed side by side with their channels aligned along the flow direction in a reactor, completely covering the cross-sectional area of the reactor, with the consequence that the gas flowing through the reactor is made to pass entirely through the channels of the monolith structures.
Honeycomb catalyst monoliths are utilised either by producing the whole monolith structure from a catalytic material, or by coating the surface of a monolith structure with a catalytically active material wherein the internal monolith structure walls contain an inert carrier material. Such monolith reactors are produced in a range of materials, typically different types of metals, ceramics or composites, wherein several production methods are known in the art. Common examples of production routes are extrusion and moulding.
Such monolith reactors can be produced with a large span in pitches and wall thickness, depending on demands on surface area, conversion, pressure drop, plugging resistance etc., as well as considerations involving monolith material strength and production limitations.
Among the advantages of monolith reactors are a low pressure drop, a relatively high surface area, reasonable production costs, and the fact that they can be utilised in processes with gas mixtures containing particulate material (dust, fly ash, soot etc.), such as effluent gases from incinerators.
The current invention concerns a novel honeycomb monolith structure having a novel honeycomb monolith channel design, especially for use in NOx-removal from exhaust/flue gases wherein the flue gas often contains particulate matter with varying particle size. Nitrogen oxides may be catalytically reduced to elementary nitrogen and water by the use of specific types of ceramic or metallic catalysts (called selective catalytic reduction, SCR). The ceramic catalysts can be extruded into a monolith structure. For the NOx removal reactions, the mass-transfer to the monolith surface is the rate-limiting step.
Common SCR catalysts are manufactured from various ceramic materials used as a carrier, such as titanium oxide, and the active catalytic components are usually either oxides of base metals, such as vanadium and tungsten, zeolites and various precious metals. Each catalyst component has advantages and disadvantages. Titanium oxide-based ceramic honeycomb SCR catalysts are often used for power generation, petrochemical and industrial processing industries.
Honeycomb monolith structures are available wherein the transversal cross section of the channels have different shapes. Such a transversal cross section is also called a cell. The most common commercially available monolith structures are honeycombs with channels having a square transverse cross section, as for example shown in International patent application WO 2012/135387 A1 (Cormetech, Inc., 2012). Also, catalytic converters with channels having a rectangular transverse cross section are known. Such a rectangular shape is, for example, disclosed in U.S. Pat. No. 5,866,080 (Day, 1999) disclosing a rectangular transverse cross section with a width/height ratio of at least 1.2, preferably in the range of 1.5 to 2.5, and in U.S. Pat. No. 6,258,436 (Siemens AG, 2001), disclosing a rectangular transverse cross section with a width:height ratio of 2:1.
Structures with hexagonal cells are also known. Chinese utility model CN201815314 relates to a honeycomb catalyst, provided with a regular hexagonal internal pore passage structure and used for SCR denitration technology. The regular hexagonal internal pore passage combines the advantages of a square internal pore passage and a circular internal pore passage. The plurality of flue gas flow internal pore passages distributed in honeycombed shapes are arranged in a square or hexagonal catalyst skeleton, and the transverse cross section of each internal pore passage is regular hexagonal, having a width:height ratio of about 1:1.
A disadvantage with the channels in prior art monolith structures is the high density of corners (corners per cm2) and/or the fact that a majority of the corners are straight corners, i.e. corners wherein two adjacent walls meet at an angle of 90 degrees. One example is the ubiquitous square channel/cell geometry.
One of the challenges with the prior art is that the corners, especially corners of 90 degrees or smaller angles, have undesirable properties, such as a low chemical conversion, a higher pressure drop and are prone to plugging and fouling with particulate material in the gas stream, with subsequent and accompanying erosion problems.
Published patent documents also exist on smoothing walls and corners in monolith structures in order to obtain a structure with an increased structural strength, as is, for example, described in US patent application 2010/0062213 A1 (Denso Corporation, 2010), which discloses an hexagonal honeycomb structure with slightly curved walls and smoothed angles between two adjacent walls, and in U.S. Pat. No. 5,714,228 (General Motors Corporation, 1999) which discloses a hexagonal shape with rounded corners.
From International patent application PCT/EP2014/051382 it is known novel monolith designs for use in mass transfer limited processes having elongated polygonal channels. Preferably the transversal cross section of the channels is hexagonal, pentagonal or octagonal. The inside corners of the channels can be rounded and all or the main part of the channels should have the same flow resistance.
As seen from this small discussion on some prior art monoliths, there are a multitude of channel configurations that have been disclosed in the prior art. Nevertheless, the ubiquitous square channels still predominates in commercial practice. In addition, most of the prior art discussed above involve automotive applications that have relatively clean exhaust emissions where plugging and fouling with particulate material in the gas stream is not a significant problem. There continues to be a need to improve channel configurations, particularly where flue gasses contain particulate matter such that the channels can become blocked over time.